The present invention relates to new hard materials with excellent properties for tools in the wood industry. More particularly, the invention relates to hard materials in which a corrosion and oxidation resistant phase has been distributed in a corrosion and oxidation resistant monophase binder based on cobalt and/or nickel and chromium.
Reconstructed wood products, such as medium density fiberboard and chipboard, are, together with solid wood, the main raw materials in the furniture industry. The are also used to some extent in the housing industry,
These products are machined with a variety of tool materials, from high speed steel to cemented carbide to polycrystalline diamond. A leading role has been and is still being played by tools made with cemented carbides.
Cemented carbide grades used for woodworking tools consist generally of WC and cobalt as a binder to hold together the WC crystals. Sometimes small amounts of other carbides are added to improve control of the grain size distribution.
Abrasion has been thought to be the primary mechanism of tool wear when machining reconstituted wood products and solid wood. Recent work has proven that chemical mechanisms such as corrosion and oxidation play a significant role in the degradation of cutting edges as the temperature increases dramatically during the machining process.
The chemical degradation of WC-Co tools is at least a two stage process when machining wood products.
At first, the degradation occurs at a low temperature (300-500.degree. C.), in the early period of cutting. As the tool temperature rises, the wood products decompose and numerous chemicals are introduced in the cutting environment. Up to 213 different compounds have been identified upon the destructive distillation of wood. The machining of medium density fiberboard and particle board produces even more decomposition products. These products contain also a binder such as urea, formaldehyde, wax and glue fillers, extenders and possibly chemicals added as flame retardants. The decomposition products formed are highly corrosive and attack the cobalt binder that holds the WC grains together. When this occurs, the WC grains are removed by chemical action and the cutting edge loses its sharpness and its cutting capability.
As the temperature rises above 500.degree. C., the decomposition products are volatilized and removed but degradation of the cutting edge continues by oxidation of the WC grains and the cobalt matrix in air. The oxides formed are readily removed by mechanical action, resulting in a fast degradation of the sharpness of the cutting edge.